1. Field of the Invention
The present invention relates to a semiconductor integration circuit, and in particular, to a differential amplifier circuit.
2. Background of the Related Art
A differential amplifier includes a non-inversion input terminal, an inversion terminal and an output terminal generating an output voltage in accordance with a differential input voltage. The differential amplifiers are used in applied fields for various purposes, one of which is a buffer. One differential amplifier used as a buffer is termed a xe2x80x98voltage followerxe2x80x99. In this buffer, an input signal is inputted to a non-inversion input terminal of the differential amplifier, and an output signal is fed back to an inversion input terminal of the differential amplifier.
FIG. 1 is a diagram that shows a related art differential amplifier circuit, which is an offset cancellation circuit of an amplifier disclosed by U.S. Pat. No. 6,049,246 (AMPLIFIER OFFSET CANCELLATION USING CURRENT COPIER). As shown in FIG. 1, the related art offset cancellation circuit detects an offset current using a current copier circuit connected to an output terminal. Then, an offset voltage is cancelled by compensating the offset voltage from an output voltage generated from a differential input voltage.
The offset cancellation circuit shown in FIG. 1 includes a current copier circuit in an output stage of a differential amplifier to detect and compensate an offset. The related art current copier circuit carries out an offset voltage detection once and stores the result. Then, the current copier circuit executes an offset compensation by applying the detected offset voltage to all output signals.
In FIG. 1, an operational transconductance amplifier (OTA) 20 is shown having input terminals 22 and 24 and output terminal 26 coupled to output node 46. A feedback path extends between output node 46 and negative input terminal 24. A first switch 56 extends between positive input terminal 22 and negative input terminal 24 for selectively shorting such input terminals together in order to null any input differential voltage thereacross. A second switch 58 is inserted within the aforementioned feedback path for selectively opening or closing the feedback path that couples output node 46 back to negative input terminal 24 of the OTA 20. When switch 58 is closed, as shown in FIG. 1, the OTA 20 operates in closed-loop fashion; when switch 58 is opened the OTA 20 operates in open-loop fashion.
As shown in FIG. 1, a current copier circuit is conceptually represented by current source 60, transistor 62, and storage capacitor 64. The current copier circuit has a first terminal 66 for selectively allowing storage capacitor 64 to be connected to the output node 46 of the OTA 20. The current copier circuit also includes a second terminal 68 coupled to the output node 46, and to the output terminal 26 of the OTA 20. The function of this current copier circuit is to xe2x80x9csupplyxe2x80x9d an offset current having a magnitude that is equal and opposite to the output offset current of OTA 20. As used herein, the term xe2x80x9csupplyxe2x80x9d could mean either sourcing current or sinking current. As shown in FIG. 1, a third switch 70 is provided for selectively coupling the first terminal 66 of the current copier circuit to the output node 46 of the OTA 20, thereby allowing the current copier circuit to respond to the voltage present on the output node 46. The current source 60 sources a fixed amount of current. The transistor 62 can be biased to sink an amount of current that is either greater than, equal to, or less than, the amount of current source by the current source 60.
As described above, the related art differential amplifier offset cancellation circuit has various disadvantages. As the magnitude of input signal of a differential amplifier varies so does that of the offset voltage included in the output voltage. Thus, the related art differential amplifier offset cancellation circuit is unable to accomplish a precise offset compensation because the identical offset voltage is applied to all of the output signals for offset compensation. Further, the offset cancellation circuit according to the related art uses a current source for detection and compensation of an offset voltage, which consumes current unnecessary for offset detection and compensation modes.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.
An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.
Another object of the present invention is to provide an offset compensation apparatus in a differential amplifier circuit and an offset compensation method thereof that substantially obviates one or more of the problems caused by limitations and disadvantages of the related art.
Another object of the present invention is to provide an offset compensation apparatus in a differential amplifier circuit and an offset compensation method thereof that compensates an offset in a differential amplifier circuit by coupling a non-inversion input terminal of a differential amplifier circuit to a storage device.
Another object of the present invention is to provide an offset compensation apparatus in a differential amplifier circuit and an offset compensation method thereof that compensates an offset in a differential amplifier circuit by coupling a non-inversion input terminal of a differential amplifier circuit to a storage device to store an offset voltage for each input signal.
Another object of the present invention is to provide an offset compensation apparatus in a differential amplifier circuit and an offset compensation method thereof that compensates an offset in a differential amplifier circuit by coupling a non-inversion input terminal of a differential amplifier circuit to a capacitor that stores an offset voltage, which is determined by detecting an offset of the differential amplifier circuit, storing the offset in the capacitor and by inputting the result of compensating the offset voltage for an input voltage into the differential amplifier.
To achieve at least the object and other advantages in a whole or in part and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention of an offset compensation apparatus in a differential amplifier circuit that drives a load includes an input stage that receives an input voltage, a differential amplifier that includes a non-inversion input terminal, an inversion input terminal and an output stage, wherein the non-inversion and inversion input terminals receive a differential input voltage and the output stage generates an output voltage in accordance with the differential input voltage, a capacitor coupled to the non-inversion input terminal, a first switch coupled between the input stage and the capacitor, wherein the first switch is controlled by a first control signal and selectively couples the input stage to the capacitor, a second switch coupled between the input stage and the non-inversion input terminal, wherein the second switch is controlled by a second control signal and selectively couples the input stage to the non-inversion input terminal, and a third switch coupled between the output stage and the capacitor, wherein the third switch is controlled by the second control signal and selectively couples the output stage to the capacitor.
To further achieve the above objects in a whole or in part, an offset compensation method in a differential amplifier circuit is provided, wherein the amplifier circuit includes an input stage that receives an input voltage, a differential amplifier having a non-inversion input terminal, an inversion input terminal and a first output stage, wherein the non-inversion and inversion input terminals receive a differential input voltage and the first output stage generates a first output voltage in accordance with the differential input voltage, a storage device coupled to the non-inversion input terminal, a first input path selectively formed between the input stage and the storage device, a second input path selectively formed between the input stage and the non-inversion input terminal, a first feed-back path between the first output stage and the inversion input terminal, and a second feed-back path selectively formed between the first output stage and the storage device the offset compensation method that includes receiving the input voltage, forming the second input path and the first and second feed-back paths to output the first output voltage that results from adding an offset voltage of the differential amplifier to the input voltage, and storing an offset voltage that is a voltage difference between the first output voltage and the input voltage in the storage device, forming the first feed-back path and maintaining a voltage level of the first output voltage, forming the first input path and the first feed-back path to input a voltage to the non-inversion input terminal that results from cancellation of the offset voltage from the input voltage by transferring the input voltage to the storage device through the first input path and outputting the output voltage equal to the input voltage by adding an offset voltage of the differential amplifier to the voltage that results by cancellation of the offset voltage from the input voltage, and forming the first feed-back path and maintaining the voltage level of the first output voltage.
To further achieve the above objects in a whole or in part, a differential amplifier circuit according to the present invention is provided that an input stage that receives an input voltage, a differential amplifier that has a non-inversion input terminal, an inversion input terminal and a first output stage, wherein the non-inversion and inversion input terminals receive a differential input voltage and the first output stage generates a first output voltage in accordance with the differential input voltage, a second output stage for connection to a load, wherein the second output stage generates a second output voltage, a storage device coupled to the non-inversion input terminal, a first switch coupled between the input stage and the storage device, wherein the first switch is controlled by a first control signal and selectively transfers the input voltage to the storage device, a second switch coupled between the input stage and the non-inversion input terminal, wherein the second switch is controlled by a second control signal and selectively transfers the input voltage to the non-inversion input terminal, a third switch coupled between the first output stage and the storage device, wherein the third switch is controlled by the second control signal and selectively transfers the first output voltage to the storage device, and a fourth switch coupled between the first output stage and the second output stage, wherein the fourth switch is controlled by the first control signal and selectively generates the second output voltage by selectively coupling the first output stage to the load.
To further achieve the above objects in a whole or in part, an offset compensation method in a differential amplifier circuit for driving a load, wherein an amplifier includes an input stage to receive an input voltage, a differential amplifier having a non-inversion input terminal, an inversion input terminal and a first output stage wherein the non-inversion and inversion input terminals receive a differential input voltage and the first output stage generates a first output voltage in accordance with the differential input voltage, a second output stage connected to a load and generating a second output voltage, a storing circuit connected to the non-inversion input terminal, a first input path formed selectively between the input stage and the storing circuit wherein the first input path transfers the input voltage to the storing circuit, a second input path formed selectively between the input stage and the non-inversion input terminal wherein the first input path transfers the input voltage to the non-inversion input terminal directly, a first feed-back path formed between the output stage and the inversion input terminal wherein the first feed-back path transfers the output voltage to the inversion input terminal, a second feed-back path formed selectively between the output stage and the storing circuit wherein the second feed-back path transfers the output voltage to the storing circuit, an output path formed between the first and second output stages selectively and transferring the first output voltage to the load, includes forming the second input path and the first and second feed-back paths, outputting the first output voltage resulted by adding an offset voltage of the differential amplifier to the input voltage, and storing an offset voltage which is a voltage difference between the first output voltage and the input voltage in the storing circuit, forming the first feed-back path, and maintaining a level of the first output voltage, forming the first input, first feed-back and output paths, inputting a voltage which is resulted by cancelling the offset voltage from the input voltage by transferring the input voltage to the storing circuit through the first input path, to the non-inversion input terminal, outputting the first output voltage equal to the input voltage by adding an offset voltage of the differential amplifier to the voltage which is resulted by cancelling the offset voltage from the input voltage, and outputting the second output voltage by transferring the first output voltage to the second output stage, and forming the first feed-back path, and maintaining a voltage level of the output voltage.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.